I guess I'm just not sold on the idea of buying a stock factory rifle for $1k when I can probably put together something special with a better barrel and a better trigger for the same price or less. Am I under-estimating? Haven't tried building a full custom before. But I have been seeing Savages listed for as low as $200 lately.
I'm thinking that if I want to reach out past 1000 yards, that I'll probably need to get into one of the 338's. My logic says to separate the two into purpose-built guns rather than compromising and doing it with one.
So what am I looking at in terms of effective range and trajectory with the 6.5-284?
How does it compare to the 7mm Rem Mag?
What about 7mm STW? Or the 7mm-08?
And what are the best long range bullets and weights for 6.5 and 7mm?
Are there some charts somewhere that I can find this information? I've only got a couple reloading manuals, but the don't give anything besides the supposed MV.
I'll try answering all your questions:
I would expect any Savage you see for $300 or less is an older version without either Accutrigger or Accustock. These can still be shooters if you get lucky, as I have an older stock Savage .243 I picked up used for a "barn gun" for my dad that put 5-shots through the same hole the first time I shot it. These are not the quality of the newer Savages that you should expect to be VERY accurate, especially in their long-range hunter configuration.
The Accutrigger on modern Savages is a technological breakthrough that gives them an accuracy advantage over ALL other rifle manufacturers. They created the first relatively inexpensive trigger that can be set very light for maximum accuracy and still be safe.
The 7 MM Rem mag is a good round that will easily do what you want, punch paper to 1,000 yards & kill deer to 800 yards, so will the 6.5-284.
The 7 MM STW and the 7 MM RUM (very similar) add considerably more powder to give you a faster bullet with a flatter trajectory than the 7 MM Rem. Each of these will take deer at 1,000 yards if you can shoot them that well. Because of the extreme amount of powder you should expect a shorter barrel life and considerably more muzzle-blast.
The 7 MM-08 is a necked down .308 cartridge that has much less velocity and energy than the 7 MM Rem mag. I consider it at most a 500-600 yard deer gun although I know others will disagree.
The 7 MM Rem mag is not a bad choice for what you are wanting to do. It's a very good choice. For the same reasons you are leaning towards it, I chose it as my first medium-game/big-game rifle long ago. I still have that 7 MM although I haven't shot it in 3 years. I think the point of the comment on the stock 6.5-284 was to let you know there are options you may not have considered, not to say you were headed a "bad" direction.
Many long-range shooters are shooting 6.5 MM / .264 bullets because of the high BC values some .264 bullets have that keep the bullets from losing energy as fast as most bullets as they travel long distances. Largely because of hand loaders and high-BC bullets the .260 Rem, 6.5-284, and .264 Win Mag are seeing a surge in popularity. (The .264 Win mag is the 7 MM Rem necked down slightly.) If I had it to do over again, my first deer/elk rifle would have been a .264 Win mag instead of a 7 MM Rem mag.
To compare cartridges you can go to any of several on-line ballistics packages, including the one on this site (That I have never used).
If you are serious about later adding a .338 mag for LONG distance shooting I would lean towards going smaller on the gun you are getting/building now and opt for a .264, whether the 6.5-284 or the .264 Win mag. Smaller will mean more comfortable and less expensive shooting.
If a later .338 is only a possibility, I think the 7 MM Rem mag, the 6.5-284 or the .264 Win mag (along with the .270 WSM) are a coin flip from each other with each having adherents and supporters.
The main reason for going custom on a LONG range rifle is to get it to handle things a stock rifle will not. The biggest issue with most stock rifles & LONG range use (other than accuracy) is that cartridges loaded with the high BC Berger bullets will not feed through most stock magazines. For this reason you'll see .270 WSM rifles built on long actions and that's why you will see semi-stock rifles with different "bottom metal".
Hope this helps!